A Brief History of c

When the Danish astronomer Olaf Roemer (Philosophical Transactions; June
25, 1677) announced to the Paris Academie des Sciences in September 1676
that the anomalous behavior of the eclipse times of Jupiter's inner moon,
Io, could be accounted for by a finite speed of light, he ran counter to
the current wisdom espoused by Descartes and Cassini. It took another quarter
century for scientific opinion to accept the notion that the speed of light
was not infinite. Until then, the majority point of view was that the velocity
of light was infinite.

The Greek philosophers generally followed Aristotle's belief that the speed
of light was infinite. However, there were exceptions such as Empedocles
of Acragas (c.450 B.C.) who spoke of light, "traveling or being at
any given moment between the earth and its envelope, its movement being
unobservable to us," (The Works of Aristotle translated into English,
W.D. Ross, Ed.; Vol. III; Oxford Press, 1931: De Anima, p. 418b and De Sensu,
pp. 446a-447b). Around 1000 A.D., the Moslem scientists Avicenna and Alhazen
both believed in a finite speed for light (George Sarton, Introduction to
the History of Science Vol. I; Baltimore, 1927; pp. 709-12). Roger Bacon
(1250 A.D.) and Francis Bacon (1600 A.D.) both accepted that the speed of
light was finite though very rapid. The latter wrote, "Even in sight,
whereof the action is most rapid, it appears that there are required certain
moments of time for its accomplishment...things which by reason of the velocity
of their motion cannot be seen-as when a ball is discharged from a musket"
(Philosophical Works of Francis Bacon; J.M. Robertson, Ed.; London, 1905;
p. 363). However, in 1600 A.D., Kepler maintained the majority view that
light speed was instantaneous, since space could offer no resistance to
its motion (Johann Kepler; Ad Vitellionem paralipomena astronomise pars
optica traditur Frankfurt, 1804).

It was Galileo in his Discorsi..., published in Leyden in 1638, who proposed
that the question might be settled in true scientific fashion by an experiment
over a number of miles using lanterns, telescopes, and shutters. The Accademia
del Cimento of Florence reported in 1667 that such an experiment over a
distance of one mile was tried, "without any observable delay"
(Essays of Natural Experiments made in the Academie del Cimento; translated
by Richard Waler, London; 1684; p. 157). However, after reporting the experimental
results, Salviati, by analogy with the rapid spread of light from lightning,
maintained that light velocity was fast but finite.

Descartes (who died in 1650) strongly held to a belief in the instantaneous
propagation of light and, accordingly, influenced Roemer's generation of
scientists, who accepted his arguments. He pointed out that we never see
the sun and moon eclipsed simultaneously. However, if light took, say, one
hour to travel from earth to moon, the point of co-linearity of the sun,
earth, and moon system causing the eclipse would be lost and visibly so
(Christiaan Huygens, Traite de la Lumiere...; Leyden; 1690, pp. 4-6, presented
in Paris to the Academie Royale des Sciences in 1678). In 1678 Christiaan
Huygens demolished Descartes' argument by pointing out, using Roemer's measurements,
that light took (of the order of) seconds to get from moon to earth, maintaining
both the co-linearity and a finite speed.

However, only Bradley's independent confirmation published January 1, 1729
ended the opposition to a finite value for the speed of light. Roemer's
work, which had split the scientific community, was at last vindicated.
After 53 years of struggle, science accepted the observational fact that
light traveled at a finite speed. Until recently, that finite speed has
been generally been considered a fixed and immutable constant of the universe
in which we live.

Scientifically speaking, the velocity of light is the highest known velocity
in the physical universe. The present value has been fixed (by definition)
since 1967 and is 299,792.458 kilometers per second. Almost everyone rounds
this off to 3 X 10 (exp8) meters/second, or 186,000 miles/second. Electronics
technicians often prefer to remember the approximate speed of light as one
foot per nanosecond in air or vacuum-the distance light travels in one-billionth
of a second. In dielectric media the velocity of light (electromagnetic
waves) is slower than in the vacuum of space.